1. Field of the Invention
The present invention relates to superconducting magnet and regenerative refrigerator mounted on the superconducting magnet and, more particularly, relates to a superconducting magnet capable of achieving easy maintenance and reduction in size and weight and to a regenerative refrigerator of an improved refrigerating capacity.
2. Description of the Related Art
FIG. 26 is a sectional view showing an example of conventional superconducting magnet, for example, disclosed in Japanese Patent Laid-Open Publication No.5-136469.
In this figure, superconducting coil 1 is contained in a coil portion helium tank 2a which serves as the coil portion cryogenic refrigerant tank. A helium reservoir tank 2b serving as the cryogenic refrigerant reservoir tank is disposed above the coil portion helium tank 2a. Further, the coil portion helium tank 2a and the helium reservoir tank 2b are in communication with each other through a helium piping 5. Liquid helium 3 serving as a cryogenic refrigerant is stored in the helium reservoir tank 2b. The interior of the coil portion helium tank 2a is filled with the liquid helium 3 which has been supplied through the helium piping 5 from the helium reservoir tank 2b. Therefore, the superconducting coil 1 contained within the coil portion helium tank 2a is immersed in the liquid helium 3 so as to be maintained at a very low temperature.
A coil portion thermal shield 8a is disposed so as to enclose the coil portion helium tank 2a, and a helium reservoir portion thermal shield 8b is disposed so as to enclose the helium reservoir tank 2b. Thus, heat penetration into the coil portion helium tank 2a and the helium reservoir tank 2b is reduced by the coil portion thermal shield 8a and the helium reservoir portion thermal shield 8b. A liquid nitrogen container 6 is filled with liquid nitrogen 7 serving as a freezing mixture and is thermally connected to the helium reservoir portion thermal shield 8b. A liquid nitrogen cooling pipe 9 is disposed such that it is wound around the coil portion thermal shield 8a in its state of thermal contact thereto. One end of the liquid nitrogen cooling pipe 9 is in communication with the bottom of the liquid nitrogen container 6 and the other end (not shown) thereof is in communication with a vapor-phase portion 6a at an upper portion of the liquid nitrogen container 6.
A vacuum tank 10 is disposed so that it furthermore encloses the coil portion thermal shield 8a and the helium reservoir portion thermal shield 8b which are disposed to enclose the coil portion helium tank 2a and the helium reservoir tank 2b. The coil portion helium tank 2a is then supported by a plurality of supports 11 adiabatically with respect to the vacuum tank 10.
A Joule-Thomson cycle refrigerator 12 for liquefying the evaporated helium gas within the helium reservoir tank 2b comprises: a compressor 13; a precooler 14 for cooling a room-temperature, high-pressure helium gas supplied from the compressor 13 by means of the helium gas which has been fed back at a low temperature and low pressure; a Joule-Thomson valve 15 for allowing an equi-enthalpy expansion of the high-pressure, low-temperature helium gas having been cooled to a predetermined temperature to occur substantially to the level of atmospheric pressure, thereby liquefying a part of the expanded gas; and a condenser 16 disposed in the vapor-phase portion at an upper portion of the helium reservoir tank 2b, for condensing and liquefying the evaporated helium gas within the helium reservoir tank 2b by the liquid helium generated at the Joule-Thomson valve 15.
A description will now be given with respect to operation of the above conventional superconducting magnet.
The superconducting coil 1 is cooled to a very low temperature (for example 4.2 K) by the liquid helium 3 within the coil portion helium tank 2a and is brought into the so-called superconductive state where electric resistance is zero. An excitation current is then supplied through a current lead (not shown) to the superconducting coil 1 from an external power supply (not shown) provided for the superconducting magnet so as to generate a required magnetic field.
The helium reservoir portion thermal shield 8b is cooled to a temperature of the order of 80 K by means of thermal conduction from the liquid nitrogen container 6 which is filled with the liquid nitrogen 7. Further, the liquid nitrogen 7 cools the coil portion thermal shield 8a by cycling through the liquid nitrogen cooling pipe 9.
Thus, heat penetration into the helium reservoir tank 2b and the coil portion helium tank 2a is reduced, since, in addition to vacuum insulation provided by the vacuum tank 10, radiated heat is cut off by the helium reservoir portion thermal shield 8b and the coil portion thermal shield 8a.
Further, the supports 11 are disposed between the superconducting coil 1 and the vacuum container 10 to bear the magnetic field generated by the superconducting coil 1 and the weight of the superconducting coil 1. Here, the supports 11 are provided with thermal anchor at the portion corresponding to the coil portion thermal shield 8a to reduce heat penetration.
However, it is impossible to completely prevent heat penetration and, as a result, the liquid helium 3 is continuously vaporized. The above described Joule-Thomson refrigerator 12 is thus driven to feed the liquid helium generated at Joule-Thomson valve 15 into the condenser 16 so as to condense and liquefy the vaporized helium gas. Thereby, evaporation of the liquid helium 3 in the helium reservoir tank 2b may be reduced, or the evaporated amount thereof may be zero.
Further, as another conventional example, a superconducting magnet as shown below has been proposed.
FIG. 27 is a cross sectional view showing another example of conventional superconducting magnet for example disclosed in Japanese Patent Laid-Open No.2-298765. In this conventional superconducting magnet, a helium tank 2 containing the superconducting coil 1 in a manner immersing it in liquid helium 3 stored therein is enclosed by a second thermal shield 17. A thermal shield 8 is disposed so as to enclose the second thermal shield 17 and a vacuum tank 10 is disposed so as to furthermore enclose the thermal shield 8. Here, Gifford-McMahon cycle refrigerator 18, a type of regenerative refrigerator operating efficiently against impurities, is used, so that the thermal shield 8 is cooled by a first-stage heat stage 19 of the Gifford-McMahon cycle refrigerator 18, the second thermal shield 17 is cooled by a second-stage heat stage 20 and, furthermore, the helium tank 2 is cooled by a third-stage heat stage 21.
The construction of the above Gifford-McMahon cycle refrigerator 18 will now be described with reference to FIG. 28.
A cylinder 31 is constructed such that pipes having sequentially reduced diameters are coaxially connected and integrated to one another. A first-stage displacer 32 is slidably disposed on the first stage of the cylinder 31, a second-stage displacer 33 is slidably disposed on the second stage of the cylinder 31 in a similar manner as the first-stage displacer 32 and a third-stage displacer 34 is slidably disposed in a similar manner on the third stage of the cylinder 31. The first, second and third-stage displacers 32, 33, 34 are connected and integrated respectively by means of universal joints (not shown). A first-stage seal 35, second-stage seal 36 and third-stage seal 37 are respectively disposed between the first, second, third-stage displacers 32, 33, 34 and the respective stages of the cylinder 31, thereby preventing leakage of helium gas. The first-stage heat stage 19, second-stage heat stage 20 and third-stage heat stage 21 are respectively disposed on the outer peripheral surface of the low-temperature end of each stage of the cylinder 31. Spaces formed respectively between the end surfaces of the respective stages of the cylinder 31 and the first, second and third-stage displacers 32, 33, 34 constitute first-stage expansion space 44, second-stage expansion space 45 and third-stage expansion space 46. A first-stage regenerator 38 is constituted by filling the interior of the first-stage displacer 32 with a copper mesh as the regenerative material. A second-stage regenerator 39 is constituted by filling the interior of the second-stage displacer 33 with lead balls as the regenerative material. A third-stage regenerator 40 is constituted by filling the interior of the third-stage displacer 34 with Ho-Er-Ru as the regenerative material.
A helium piping for supplying/exhausting helium gas is attached to the Gifford-McMahon cycle refrigerator 18. A suction valve 41 is mounted on the supplying side of the helium piping as a valve mechanism, and timing for supplying a high-pressure helium gas compressed at the compressor 13 to the Gifford-McMahon cycle refrigerator 18 is controlled by the suction valve 41. An exhaust valve 42 is mounted on the returning side of the helium piping as a valve mechanism, and timing for exhausting the low-pressure helium gas to the compressor 13 from the Gifford-McMahon cycle refrigerator 18 is controlled by the exhaust valve 42. A driving motor 43 causes reciprocation of the first, second and third-stage displacers 32, 33, 34 within the cylinder 31. The suction valve 41 and the exhaust valve 42 are opened/closed in association with such reciprocating movement.
Operation of the Gifford-McMahon cycle refrigerator 18 constructed as described is as follows.
First, in the state where the first, second and third-stage displacers 32, 33, 34 are placed at the lowermost end and where the suction valve 41 is opened and the exhaust valve 42 is closed, a high-pressure helium gas compressed at the compressor 13 is supplied into the first, second and third-stage expansion spaces 44, 45, 46. As a result, a high-pressure state occurs in the first, second and third-stage expansion spaces 44, 45, 46.
Next, the first, second and third-stage displacers 32, 33, 34 are moved upward, and, accordingly, the high-pressure helium gas is sequentially supplied to the first, second and third-stage expansion spaces 44, 45, 46. In the meantime, the suction and exhaust valves 41, 42 are not moved. Thus, the high-pressure gas is cooled to a predetermined temperature by the respective regenerative materials when it passes through the first, second and third-stage regenerators 38, 39, 40.
When the first, second and third-stage displacers 32, 33, 34 reach the uppermost end, the suction valve 41 is closed and, shortly thereafter, the exhaust valve 42 is opened. At this time, the high-pressure helium gas is adiabatically expanded to cause refrigeration. The helium gas existing within the first, second and third-stage expansion spaces 44, 45, 46 is then brought to a low-temperature and low-pressure state at the respective temperature level.
Next, as the first, second and third-stage displacers 32, 33, 34 are moved downward, the low-temperature and low-pressure helium gas passes through the third, second and first-stage regenerators 40, 39, 38 and is exhausted from the exhaust valve 42. At this time, after cooling the regenerative materials respectively of the third, second and first-stage regenerators 40, 39, 38, the low-temperature and low-pressure helium gas is returned to the compressor 13.
Then, in the state where the first, second and third-stage displacers 32, 33, 34 are moved to the lowermost end to minimize the volume of the first, second and third-stage expansion spaces 44, 45, 46, the exhaust valve 42 is closed and the suction valve 41 is opened so that, as the high-pressure helium gas compressed at the compressor is supplied, the pressure of the first, second and third-stage expansion spaces 44, 45, 46 is increased from a low pressure to a high pressure. The above process constitutes one cycle of operation.
In this manner, the above operation is repeated so that temperatures of the first, second and third-stage heat stages 19, 20, 21 are cooled to 70 K, 20 K, 4.2 K, respectively.
While the above description has been given with respect to a 3-stage type Gifford-McMahon cycle refrigerator 18, operation of a 2-stage type Gifford-McMahon cycle refrigerator is similar to the 3-stage type Gifford-McMahon cycle refrigerator 18 with an only exception that number of displacers, regenerators, seals and expansion spaces is changed from three to two, respectively. Here, if the above operation is repeated with a 2-stage type Gifford-McMahon cycle refrigerator, the first and second-stage heat stages are cooled to 50 K and 4.2 K, respectively.
As described, in the conventional superconducting magnet such as disclosed in Japanese Patent Laid-Open No.5-136469, since the coil portion thermal shield 8a and the helium reservoir portion thermal shield 8b are cooled by liquid nitrogen 7, replenishment of the liquid nitrogen 7 is required at suitable intervals. Since the liquid nitrogen container 6 must be provided, the superconducting magnet is increased in size to result in the problem of an increased weight thereof.
Further, since the coil portion thermal shield 8a and the helium reservoir portion thermal shield 8b cannot be brought to temperatures lower than the temperature of the liquid nitrogen, heat penetration due to conduction or radiation into the coil portion helium tank 2a and the helium reservoir tank 2b depends on the boiling point temperature (77 K) of liquid nitrogen. As a result, there is a limitation in reducing the evaporating amount of liquid helium 3 within the coil portion helium tank 2a and the helium reservoir tank 2b.
Further, since the container for storing liquid helium 3 is divided into two locations at the coil portion helium tank 2a and the helium reservoir tank 2b, the thermal shield is constituted by the coil portion thermal shield 8a and the helium reservoir portion thermal shield 8b, resulting in a problem of heat resistance of the connecting portion when the two parts are thermally integrated through a heat transmission member. For this reason, if a conventional regenerative refrigerator is provided at the portion of the coil portion thermal shield 8a, the helium reservoir portion thermal shield 8b will not be adequately cooled and temperature thereof will be increased, whereby heat penetration into the helium reservoir tank 2b is increased. Further, if the conventional regenerative refrigerator is provided at the helium reservoir portion thermal shield 8b, the coil portion thermal shield 8a will not be adequately cooled and the temperature thereof will be increased, whereby heat penetration into the coil portion helium tank 2a is increased. In either case, problem occurs of an increased evaporation of liquid helium 3.
Further, while the Joule-Thomson cycle refrigerator 12 is provided to reduce the evaporating amount of liquid helium 3, clogging due to a small amount of impurities of the helium gas serving as the working fluid tends to occur at the small bore portion of the Joule-Thomson valve 15 in the Joule-Thomson cycle refrigerator 12. There are problems not only of difficulty of handling but also of higher costs due to the fact that the Joule-Thomson cycle refrigerator 12 itself has a complicated construction.
On the other hand, in the conventional superconducting magnet disclosed in Japanese Patent Laid-Open No,2-298765, the Gifford-McMahon cycle refrigerator 18 which is efficient in dealing with impurities is used to reduce the evaporating amount of liquid helium 3. Therefore, clogging due to a small amount of impurities included in the helium gas is less likely to occur compared to the conventional superconducting magnet using the Joule-Thomson cycle refrigerator 12. Handling is easier and the construction is simpler whereby lower cost may be achieved. With the Gifford-McMahon cycle refrigerator 18, however, the most suitable cycle frequency (number of cycles per unit time) differs between the temperature of the order of 4 K) at the time of liquefying helium gas and the temperature (10 K or higher) for cooling the thermal shield. Thus, if a cycle frequency suitable for cooling the thermal shield is adopted, capacity for liquefying helium gas is lowered. On the other hand, if a cycle frequency suitable for liquefying the helium gas is adopted, the cooling power for the thermal shield is reduced, whereby heat penetration into the helium tank 2 is increased to result in an increased evaporation of liquid helium 3.
Accordingly, the liquefying capacity for helium gas in the Gifford-McMahon cycle refrigerator 18 is not adequate and it is necessary to improve the liquefying capacity.